Effect of algae- and silt concentration on clearance- and growth rate of the razor clam Ensis directus, Conrad

Laboratory experiments were carried out with Ensis directus to estimate clearance rate and growth rate as a function of the amount of food and presence of silt. Two food levels were tested: low food availability (6.5μg chla/l) and high food availability (16.5μg chla/l) at four silt concentrations (0, 50, 150 and 300mg/l). Clearance rates of E. directus varied between 0.7 and 5.9l/h/g DW. At a silt concentration of 300mg/l clearance rates were significantly lower (16–56%) than rates at 150mg/l, 50mg/l and 0mg/l silt. This indicates that only the highest silt concentration induced a reduction in clearance rate. The tested food levels did not influence the clearance rate of E. directus. Maximum growth rate of E. directus was 0.24mm increase in shell length and 2% increase in wet weight per day. Long-term (10weeks) exposure to silt concentrations of 300mg/l showed significantly higher growth than the 150mg/l treatment. This indicates that exposure to a high silt concentration did not induce a reduction in growth, but stimulated growth. Long-term (10weeks) exposure to a food level of 6.5μg chla per litre reduced the shell growth of E. directus with 40% compared to growth at 16.5μg chla per litre. The laboratory experiments suggest that E. directus is more sensitive to a reduction in algal concentration than to an increase in silt concentration. •Silt concentration affected filtration rate, food concentration did not•Long-term exposure to silt no reduction in growth•Highest growth rates observed at the highest food level